methicillin-resistant Staphylococcus aureus

methicillin-resistant Staphylococcus aureus
Name	methicillin-resistant Staphylococcus aureus
Synonyms	MRSA
Field	Infectious disease (medical specialty), Microbiology
Causes	Staphylococcus aureus

methicillin-resistant Staphylococcus aureus. Methicillin-resistant *Staphylococcus aureus* (MRSA) is a significant pathogen responsible for difficult-to-treat infections in healthcare and community settings. Its emergence represents a major challenge in modern medicine due to its resistance to many beta-lactam antibiotics. The organism's ability to cause a wide spectrum of disease, from minor skin conditions to life-threatening sepsis, makes it a persistent public health concern globally.

Overview

The first isolates were identified shortly after the introduction of methicillin in the early 1960s, with reports from the United Kingdom. MRSA infections are broadly categorized into two types: healthcare-associated (HA-MRSA) and community-associated (CA-MRSA), each with distinct epidemiological and genetic profiles. The global spread of these strains has been documented by organizations like the World Health Organization and the Centers for Disease Control and Prevention. The economic and human cost of MRSA is substantial, impacting hospitals from Johns Hopkins Hospital to facilities across Europe and Asia.

Microbiology and resistance mechanisms

*Staphylococcus aureus* is a gram-positive coccus that commonly colonizes the skin and nasopharynx. Resistance to methicillin and other beta-lactams is primarily mediated by the *mecA* gene, which is carried on a mobile genetic element called the staphylococcal cassette chromosome *mec* (SCC*mec*). This gene directs the production of penicillin-binding protein 2a (PBP2a), which has a low affinity for beta-lactam antibiotics. The transfer of SCC*mec* elements is a key factor in the evolution of resistant clones, such as the historically significant EMRSA-15 and EMRSA-16 strains in the UK.

Epidemiology and risk factors

Epidemiology varies significantly between HA-MRSA and CA-MRSA. HA-MRSA is traditionally associated with exposure to healthcare environments like intensive care units, surgical wards, and long-term care facilities such as nursing homes. Risk factors include invasive procedures, indwelling devices like central venous catheters, and prolonged hospitalization. CA-MRSA emerged distinctly in the 1990s, with notable outbreaks among otherwise healthy individuals in places like Los Angeles, Texas, and among prison populations. The USA300 clone is a predominant CA-MRSA strain in North America.

Clinical presentation and diagnosis

Clinical manifestations range from mild folliculitis and abscesses to severe conditions like pneumonia, endocarditis, and osteomyelitis. HA-MRSA is frequently implicated in surgical site infections and bacteremia. Diagnosis requires microbiological culture and antimicrobial susceptibility testing. Key laboratory methods include the use of chromogenic agar and testing for oxacillin resistance. Molecular techniques like polymerase chain reaction (PCR) for the *mecA* gene, used in laboratories like those at the Mayo Clinic, provide rapid confirmation.

Treatment and management

Treatment is complicated by multidrug resistance. While vancomycin has long been the cornerstone therapy for serious infections, the emergence of strains with reduced susceptibility, such as vancomycin-intermediate *Staphylococcus aureus* (VISA), has prompted the use of alternative agents. These include linezolid, daptomycin, ceftaroline, and tedizolid. The Infectious Diseases Society of America publishes guidelines for management. Incision and drainage of abscesses is a critical component of therapy for skin and soft tissue infections.

Prevention and control

Infection control is paramount, especially in institutional settings. Core strategies include strict adherence to hand hygiene protocols using alcohol-based hand rubs, contact precautions for infected or colonized patients, and environmental cleaning. Screening programs, such as those implemented in the Netherlands (the "Dutch search and destroy policy"), aim to identify carriers. Decolonization regimens may involve mupirocin ointment and chlorhexidine washes. Research into vaccine development continues at institutions like the National Institutes of Health.

Category:Infectious diseases Category:Antimicrobial resistance